Stencil printing with vacuum support frame

ABSTRACT

A stencil manufacturing and printing process reduces printed image distortion by matching screen deflection in the photostencil preparation phase with stencil deflection in the printing phase. The usual squeegee is replaced by atmospheric pressure and screen/stencil deflection is induced by producing a vacuum through a specially constructed air distribution frame. Compressed air can be introduced through the same frame to rapidly release the stencil from the substrate when the printing is completed. A control unit incorporating a pressure sensor facilitates control of the printing process. The frame may have a 2-piece construction with the stencil/screen supported by a removable insert and the air distribution and pressure level sensing functions afforded by a master frame into which the insert fits.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.667,633 filed Nov. 2, 1984, now abandoned U.S. Pat. No. 4,649,817.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates, in general, to "silk scree" printing and moreparticularly to an improved process and new apparatus for manufacturingand printing with a stencil which provides greater accuracy and speedand significantly reduced distortion.

2. Background Information

"Silk screen" printing is an old and well established art which employsa screen supported stencil to provide a dense and opaque layer of ink ona substrate. The name "silk screen" comes from the threads originallyused to support the various individual elements of the stencil. Today,this printing process enjoys widespread commercial application and isused to print on such diverse items as dishes, dials, poster board,plexiglass sheets, textiles and silicon wafers. In the electronicsindustry the process is sometimes referred to as thick-film printing.

The process involves two phases: stencil manufacture and stencilprinting. In the manufacture of a stencil, the screen fabric (nowgenerally polyester, not silk) is stretched tightly across a stableframe to which it is adhered. This stretched screen is then coated witha photosensitive emulsion. A film positive, containing an opaque imageon a clear film base of the art to be printed, is placed in contact withthe bottom of the emulsion coated screen. Both are then placed in avacuum frame consisting of a glass plate and a rubber blanket. Air iswithdrawn from between the glass and the rubber sandwiching the screenand contiguous positive between the two. U.S. Pat. No. 3,463,587illustrates such a vacuum frame used for preparing silk screen stencils.

The emulsion coated screen is photo exposed by beaming a strong lightthrough the glass of the vacuum frame to harden all emulsion not maskedfrom the light by the opaque lines of the film positive. The exposedscreen is then removed from the vacuum frame to be dampened by a sprayof water that dissolves all unhardened areas completing the stencilmaking process.

The stencil is now ready for printing. This generally requires, inaddition to the ink and the substrate to be printed on, three basicappliances. The first is a table to support the stencil and thesubstrate to be printed on. The table is commonly perforated andattached to a vacuum to hold the substrate securely during the printingprocess. The second appliance is a hinge/clamp device that attaches thestencil frame to the table so that it may be raised and lowered to theexact same position each time a new substrate is placed on the table forprinting. The third commonly used appliance is the squeegee, a resilientscraping device that spreads the ink across the back of the stencil andapplies the pressure that causes the ink to pass through the stencilonto the substrate.

With these basic elements, a number of complex mechanisms have beenconstructed. These include the hand-operated, semi-automatic,three-quarter automatic, fully automatic, as well as cylinder and rotaryscreen printers. With the exception of the highly specialized rotaries,these prior printers all generally utilize the frame stretched stenciland the squeegee. The cylinder press has a drum bed (table) instead of aflat bed and its stencil reciprocates in a motion with the drum bedinstead of hinging or rising to allow the replacement of a substrate.

In general, the screen printing process employs the following sequenceof steps: after a register is determined and all adjustments have beenmade (called set-up) an amount of ink is placed on the stencil outsideof the image area, as wide as the image, and a squeegee slightly widerthan the image is placed behind the ink, pressed down and moved witheven pressure across the image area forcing the ink through the openstencil spaces and onto the substrate. The screen is then lifted and theink is pushed back over the image area with little pressure returning itto the point of origin and "flooding" the screen in the process. Whilethe screen is raised, the substrate is released and removed for dryingand another substrate is placed in registry on the table so the sequencecan begin again.

With the exception of rotary screen printers, the sequence describedabove is generally used by all mechanisms that use a screen/stencil forprinting. Variations may occur as system options such as flood bars thatreturn the ink by lifting and carrying it instead of coating the imagearea; or, as in the case of the cylinder press, a stationary squeegeemay be made to traverse the image area by moving the stencil and the bedin unison relative to a stationary squeegee. This same variation is usedwhen cylindrical objects such as bottles or glasses are stencil printed.On the whole, though, adaption or modification of the individualcharacteristics of the basic sequence does not change the function ofthe sequence. The ink is still forced through a frame-supported stencilby a squeegee to produce the desired impression.

A brief description of some basic characteristics of screens and thescreen/stencil printing process is helpful in understanding the wideapplication of this technology. First, the screens that support thestencils are woven in a range of fineness from as coarse as 16 threadsper inch, with a thread thickness of 0.0138 inches to as fine as 1735threads per inch with a thread thickness of 0.0008 inches. Since theemulsion coating completely encapsulates the screen and the ink isdeposited relative to the emulsion thickness, the thickness of theimpression from the stencil is equal to a calculable amount based on thethread thickness and the ink film thickness after drying. Inapplications such as electronic circuit printing or plating and solderresist printing, this ability to control film thickness for functionalpurposes is a practical and economic use of stencil printing.

The nature of this printing technique also contributes to its wideapplication. In its most basic form, screen/stencil printing requiresnothing more than a screen supported stencil and squeegee to produce animpression. No mechanism to actuate pressure is needed. For this reason,stencils may print virtually any size without requiring a machine ofcorresponding dimension. It is only necessary to be able to sufficientlycontact the object to be printed. The impression is made by fluidpressure of the ink and the surface attraction of the material under thestencil. Thus screen/stencil printing is versatile enough to print onlarge, solid objects and minute, delicate objects with virtually thesame pressure.

Together, these characteristics make "silk screen" printing a uniqueprinting process fulfilling product demands that establish it as anessential technology in contemporary manufacturing. Nevertheless, theexisting process suffers from significant limitations; the most notableof which is image distortion.

The inventor has identified two primary sources of printed imagedistortion occurring in the existing screen/stencil printing process.The first arises from the use of a squeegee transversing a stencil toapply an image to a substrate. When this instrument is drawn across thestencil-supporting screen fabric an amount of friction-produced stretchand accompanying image elongation is inevitable. Further, the squeegeeproduces undesirable vibration, and static electricity. The latter canattract dust and other particles in the air producing a glitch in theprinted image.

The second principal source of distortion arises because the stencilmust be supported a slight distance above the substrate in the printingstage to permit the necessary "peel" of the stencil behind the squeegeethat assures good edge definition in the printed image; while, in thephotoexposure process, the screen fame is held in planar contact withthe glass during the vacuum hold. Since the image is exposed at onelevel and deflected to another for printing, an additional degree ofdistortion is also inevitable.

Attempts have been made in the past to eliminate the squeegee from thestencil printing process. See for example, U.S. Pat. Nos. 3,172,358,3,221,648 and 3,221,649 to F. Weiss. These patented devices employ avacuum induced through the printing bed to suck ink through the stenciland onto a substrate. This process, however, appears to require a poroussubstrate which must be prewet with solvent. These requirements coupledwith the convex printing bed and the absence of any mechanism forquickly releasing the stencil from the substrate, makes this apparatusunsuitable for fine, close tolerance, printing applications. Notefurther that there is no recognition in these patents of the secondprincipal source of image distortion discussed above and that thepatented structure is inherently incapable of addressing the secondproblem.

U.S. Pat. No. 3,964,385 is directed to a "Unitary Device and Method ForScreen Manufacture and Printing" but fails to recognize and redress theimage distortion problems discussed above.

When close tolerances and minimum distortion are required in the screenprinting process, for example, in electronics applications, a number ofcumbersome procedures have been employed to overcome the distortionproblems. These have entailed much time consuming analysis and artmodification.

A need thus persists for a stencil printing process and apparatus whichcan effectively overcome the above described drawbacks of the existingtechnology.

SUMMARY OF THE INVENTION

Briefly, the present invention satisfies this need by totallyeliminating the squeegee from the printing process and employing thescreen/stencil as a diaphragm which is equally deflected in the stencilmanufacturing and stencil printing stages. Through the selectiveapplication of vacuum pressure, the image area of the screen isdeflected to contact the film positive in the stencil manufacturingprocess and the ink bearing stencil is equally deflected to contact thesubstrate in the stencil printing process. Use of the same airdistribution frame dimensional parameters (i.e. peripheral off-contactdistance) in inducing both deflections assures precise control overimage stability. The invention also contemplates use of a special screenfinishing process in the stencil manufacturing phase to further controlprinting characteristics. Unique control units incorporating acompressed air stencil release mechanism, vacuum level sensor andcontrol circuitry are designed to further facilitate rapid, accurate anddistortion free printing.

In a preferred embodiment, the frame has a two-piece construction andincludes a master frame providing the air distribution function and adisposable insert which supports the screen/stencil. The master frame isconfigured to receive the cooperating insert and support the insert sothat the periphery of the screen/stencil is maintained at apredetermined off-contact distance. The control unit preferablyincorporates a manifold absolute pressure sensor which detects bothvacuum and positive pressure levels, and solid state logic circuitrywhich performs rapid and accurate control function signal processing.

Accordingly, a principal object of the invention is to provide a methodand dual purpose stencil manufacturing and printing apparatus whicheliminates the primary sources of image distortion in stencil printing.

Another object of the invention is to provide a stencil preparation andprinting process and apparatus which is safe, simple, versatile and costeffective as well as precise and controllable.

Yet another object is to provide such a process and apparatus whicheliminates the disadvantages associated with a squeegee while improvingthe accuracy, definition, and image stability of the final printedimage.

A further object of the invention is to provide a new and uniqueapproach to "silk screen" printing which correlates and integrates thestencil preparation and stencil printing phases to reduce final imagedistortion.

A still further object is to provide a screen/stencil supporting framewith an air distribution function that can be employed to selectivelyand controllably deflect the screen or stencil supported by the frame.

Another object is to provide a stencil supporting, air distributionframe which can easily be constructed from readily available materials,in varying sizes and configurations, and advantageously employed in awide variety of screen printing applications to print on diverseobjects.

Another object is to provide a compact, portable, adjustable, automatic,high-speed and accurate control unit for precisely controlling thedeflection of a screen for photo exposure purposes, the deflection of astencil for printing purposes and/or the deflection of any diaphragm forany desired purpose.

Yet another object of the invention is to provide a special screenfinishing process, two-piece frame construction, printing processcontrol unit and stencil rapid release mechanism and other refinementswhich further facilitate fast, close tolerance, substantially distortionfree printing.

Still another object is to provide improvements in both the stencilpreparation and stencil printing processes which when integrated producea superior process and product.

A further object is to reduce the cost, operator training time and skilllevel, and set up and clean up times and enhance the speed and qualityof the stencil printing process.

Another object is to provide apparatus for implementing the improvedprocesses which can be embodied as an add-on to existing thick film orflat bed screen printers to upgrade that equipment for greater precisionand higher production rates and to facilitate more cost effectivecompliance with workplace safety standards.

A further object is to provide a printhead and associated control unitwhich can be advantageously applied to a vast variety of selectivedeposition needs to precisely control the depth and/or resolution and/orcomposition of a deposit and which is applicable to: decorative art;printing of alphanumerics, bar code symbology and other markings;manufacturing of electronic components such as printed circuits andmembrane switches; and dispensing of material such as adhesives,pharmacological and biomedical substances.

These and other objects, advantages and features of the invention willbe more readily apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view from the top of one embodiment of the airdistribution frame (partially cut away) of the present invention;

FIG. 2 is a cut-away side view of a portion of the frame of FIG. 1resting on a supporting surface;

FIG. 3 is a perspective, partially cut-away view of the frame of FIG. 1and associated equipment employed in the stencil manufacturing phase;

FIG. 4 is a depiction, in magnified side view, of the photostencilmanufacturing process of the invention;

FIG. 5 is a partially cut-away view of the frame of FIG. 1 and anassociated control unit of the present invention employed in the stencilprinting phase;

FIG. 6 is a depiction in side view of the stencil printing process ofthe present invention;

FIG. 7 is a schematic of the control unit shown in FIG. 5;

FIG. 8 is a perspective view from the top of a frame or printhead of thepresent invention having a two-piece construction;

FIG. 9 is an enlarged cut-away side view of a portion of the frame ofFIG. 8;

FIG. 10 depicts the manner in which a screen/stencil supporting insertis mounted within a master frame of a printhead;

FIG. 11 is a sectional view of the printhead of FIG. 8;

FIG. 12 is a perspective, partially cut-away view of the frame of FIG. 8in conjunction with a preferred form of the control unit of the presentinvention employed in the stencil manufacturing phase; and

FIG. 13 is an electrical schematic of the control unit of FIG. 12.

DETAILED DESCRIPTION

Although departing in several fundamental respects from existingpractice, the present invention follows the general steps of the "silkscreen" process described previously. For purposes of discussion, it isconvenient to divide this process into a stencil preparation ormanufacturing phase and a stencil printing phase. Previously thesephases have generally been considered as separate, independent andunrelated processes. The present invention is unique in that itcorrelates and integrates these two phases by matching screen deflectionoccurring in the photostencil preparation phase with the stencildeflection of the printing phase. This is optimally accomplished throughthe use of a common vacuum inducing, screen/stencil supporting frame,hereinafter sometimes referred to as an air distribution frame orprinthead. The term "screen/stencil" and "screen member" are usedinterchangeably herein to genericly refer to the screen employed in themanufacturing phase and the stencil employed in the printing phase.

A first embodiment of the air distribution frame of the presentinvention is depicted in perspective view in FIG. 1. As illustrated inthis figure, air distribution frame 10 is of generally rectangularconfiguration. Frame 10 surrounds a central opening 12 across which ascreen 14 may be tautly stretched in a manner well known in the art.Screen 14 is peripherally secured to the underside of frame 10 asillustrated in the cut-away view on the right side of FIG. 1. Stretchedscreen 14 may be adhered to frame 10 by tape, adhesive or any othersuitable means. The bond between the screen and frame should besufficiently strong to allow for repeated deflection of screen 14 out ofa first plane in which it is normally held by the frame. Screen 14includes an image area 16 overlying at least a portion of centralopening 12. Image area 16 which may be of any desired size and shape isgenerally coextensive with the source artwork.

Referring now to both the top view of FIG. 1 and the cut-away side viewof FIG. 2, frame 10 preferably includes a first frame member 20extending around and away from central opening 12 and an upright secondframe member 22. The frame is thus, preferably, of angular cross sectionwith the lower edge of second frame member 22 adjacent the inner edge offirst frame member 20, and the frame members meeting at substantially90°.

The frame may be constructed of angle iron cut to length and mitered (asillustrated by dashed line 25 in FIG. 2) to form a frame of any desiredsize and shape. Alternatively the frame may be of a unitary constructionformed by casting or other known techniques. The frame is preferablymade of lightweight, strong material such as aluminum or plastic whichretains its structural integrity when formed and employed as a screensupporting structure.

First frame member 20, in effect, defines a continuous peripheral bandextending around central opening 12. The lower surface 24 of framemember 20 provides the mounting surface for screen 14. Outer edge 26 offrame member 20 preferably serves as a mounting rim for a peripheralgasket 28. The nominal outer corners of frame member 20 are radiused toan even curvature thereby facilitating the attachment of gasket 28 tothe outer cornerless border of the band defined by frame member 20.Gasket 28 may comprise slit soft rubber or latex tubing suitably securedto outer edge 26 and lapping under lower surface 24 of frame member 20.The gasket serves to provide a continuous airtight peripheral seal underframe 10. So long as it extends below lower surface 24 and serves thisfunction, the gasket may be formed of other materials, possess othershapes, and be secured at other locations to frame member 20.

Upright frame member 22 lends rigidity and strength to the frame andalso provides attachment points for auxiliary clamps or the like whichmay be used to lift or pivot the frame. The upright frame member maytake other forms and shapes and may be discontinuous or, underappropriate circumstances, dispensed with altogether.

Attached to frame member 20 inside the gasket area but outside ofupright member 22 is a hose connector or port 30. As shown in FIG. 2,port 30 is a conventional, commercially available hose connectorpreferably screwed into a threaded aperture extending through framemember 20. Port 30 serves to connect the airtight chamber formed underframe 10 to an external source of pressure as more fully explainedhereinafter. Depending upon the size of frame 10 and the capacity of theexternal pressure source, multiple ports 30 may be provided on theframe.

An additional hose connector or port 32 also in pneumatic communion withthe chamber or space under frame 10 is utilized in the stencil printingphase. Port 32 and its associated aperture in frame member 20 may beadded after the stencil is manufactured or simply sealed during thisinitial phase.

As generally illustrated in FIG. 3, for stencil manufacturing purposes,the stretched screen 14 (at a minimum, the image area thereof) is coatedwith a photosensitive emulsion 34 and any peripheral areas of the screenwhich remain gas previous are sealed, for example, with tape 36. Theframe is then placed on a transparent base, e.g. glass plate 38 of aphotoexposure table, in such a way that an emulsion coated image area 39of the screen overlies a film positive 40 comprising the source artwork

Hose 42 from a vacuum source or pump 44 associated with a pressure gauge46 and a bleeder valve 48, is now attached to frame 10 via port 30 andany aperture in the frame associated with port 32 is sealed, as shown at50. The vacuum pump is engaged and the screen is drawn down pressingagainst the film positive. Bleeder valve 48 is adjusted so that the drawof the screen is substantially just to the edge of film positive 40 andnot excessively beyond it toward the frame, as shown in FIG. 4. Thispressure is observed on gauge 46 and is noted for later use in theprinting phase. The noted vacuum pressure is, of course, a measure ofthe deflection of screen 14 from a first plane (shown in phantom in FIG.4) in which it is supported by frame 10, to a position in which theimage area fully contacts the film positive. Gasket 28 is preferablymade of low durometer but substantially unyielding material permittingthe gasket to remain firmly seated during air evacuation whilemaintaining the periphery of the screen at a desired off-contactdistance relative to plate 38.

With the screen in its deflected condition, the image area can bephotoexposed, in conventional manner, by a light source 52. The raysfrom this source pass through glass plate 38 and the non-opaque areas offilm positive 40 to strike contiguous portions of the screen image area.After photoexposure, the vacuum pressure under the frame is removedallowing the deflected screen to return by atmospheric pressure to itsinitial frame supported position. The frame supported screen is thenremoved from the exposure table and the image developed and dried andany necessary touch-up done to produce the desired stencil.

FIGS. 5-7 illustrate how the stencil mounted on the same airdistribution frame (or one peripherally supporting the stencil at thesame off-contact distance) is advantageously employed, according to thepresent invention, in the stencil printing phase.

In FIG. 5 the stencil developed from screen 14 is identified byreference character 54. The developed screen is no longer sealed to airtransmission. For printing, the open pores 56 of the stencil 54 aresealed by coating them with a viscous ink 58 that is used to make theimpression 60 on a substrate 62 supported on a printing table or platen64. Substrate 62, although depicted, for illustrative purposes as a thinflat sheet may take on a different shape or form depending upon theparticular application. For instance, in the electronics industry, thesubstrate might be a silicon or other semi-conductor wafer on which avery fine pattern of conductive material is to be deposited. Similarly,the viscous ink may comprise any one of the multitude of knowncompositions depending upon the particular application.

For printing purposes, frame 10 is connected by port 30 and a hose 66 toa port (not shown) of a control unit 68. The control unit is in turnconnected via ports 70 and 72 and hoses 74 and 76 to a vacuum source 78and a compressed air source 80, respectively. Port 32 connects the framethrough tube 82 and a port (not shown) on control unit 68 to a vacuumsensing switch S2 within the control unit.

Control unit 68 (which is depicted with its top cover removed) isequipped with a vacuum valve V1, a compressed air pressure valve V2, andinternal hoses for connecting vacuum valve V1 to port 70 and hose 66,and connecting compressed air valve V2 to port 72 and hose 66. Thecontrol unit also includes a set of three timers, T1, T2 and T3, threeassociated timer adjustment knobs (i.e. potentiometers RH1, RH2 and RH3,three corresponding indicator lights, L1, L2 and L3, a master powerswitch S3, and a three position, mode selection switch S1. The controlunit further includes a vacuum pressure adjustment knob 83 coupled tovacuum sensing switch S2 for setting this switch to a particular vacuumpressure level. Gauge 84, connected to hose 82, measures the actualvacuum pressure under frame 10. Knob 86 allows for adjustment ofcompressed air pressure which is measured by pressure gauge 88. As shownin FIG. 5, related knobs, lights, gauges, etc. are preferably grouped byfunction on control panel 89 of control unit 68. Finally, control unit68 contains a set of relays R1 and R2 and electrical wiring. Theoperation of the various components of the control unit will be fullydescribed, hereinafter, in connection with the schematic of FIG. 7.

The operation of the air distribution frame and associated control unitin the stencil printing stage is depicted in FIG. 6. As shown therein,the stencil 54, laden with ink 58, again acts as a diaphragm; this timemoving the ink down to the substrate 62 to make an impression and thenreturning it back to its rest position (shown in phantom) after thecontact. This is accomplished by alternating vacuum draw and compressedair release through the frame, via hose 66, to the gasketed, sealed areaunder the control of control unit 68. The same pressure used in theexposure stage is employed to deflect the stencil into contact with thesubstrate to print thereon.

In operation, vacuum sensor switch S2 is adjusted to detect and triggerat a pressure level equal to the reading observed during the exposurephase of the stencil manufacturing process. The substrate 62 to beprinted on is secured mechanically or by means of a vacuum holddown 90to the printing platen 64. When the stencil supporting frame 10 islowered to seal on the printing platen, the printing cycle begins. Thevacuum pressure under the stencil is measured through the sensor tube 82and when the predetermined vacuum pressure level is reached, vacuumsensitive switch S2 activates vacuum timer T1 which is adjusted tocontrol the deposit of ink. At the end of a first preselected period oftime, the vacuum valve V1 is closed and the compressed air valve V2 isopened in time to permit entry of enough air for the rapid release ofthe stencil from its printing position. The compressed air induces arapid off contact release which is important in producing a cleanimpression. The use of matching deflections in the stencil manufacturingand stencil printing phases and the elimination of the squeegee in theprinting process overcomes the two primary sources of image distortionthereby resulting in a much more accurate reproduction of the originalimage.

The operation of control unit 68 will now be described with reference tothe electrical schematic of FIG. 7. In general the control unit providestimed control over the compressed air pressure and vacuum used in thestencil printing phase of the present invention in both manual andautomatic printing operations. The process proceeds through thefollowing steps: the print cycle begins with a valve V1 supplying vacuumto draw down the stencil; when the predetermined vacuum pressure levelis detected, a timer T1 commences timing the vacuum print interval andat its conclusion closes the vacuum valve and begins the timed durationopening of a pressure valve V2 to rapidly release the stencil from thesubstrate and also begins a timed duration interrupt after which thenext print cycle will be allowed to begin. This last step preventsunintended repeated overprinting which would otherwise be possible withthe automatic cycling. Instead, the control unit provides for a pausebefore the next print cycle can be initiated. It should be noted thatthe duration of all cycles is fully adjustable by means of timer knobs(i.e. potentiometer settings) and that the operation of each cycle is"announced" by a pilot or indicator light adjacent to the respectiveknob.

In the manual printing mode, the printing cycle is initiated by theoperator pushing a spring loaded momentary switch; in automaticoperation, the sealing of the stencil frame bottom gasket and theappearance of vacuum initiates the process. A detailed explanation ofthe operation of the control unit follows.

The circuits in the control unit are energized by turning master powercontrol switch S3 on. A second switch S1, which is a single-pole,double-throw type switch with a center-off position, is then activated.When switch S1 is deflected to its upper momentary contact position, themanual printing process begins. Depression of switch S1 to its lowermaintained contact position initiates the automatic printing process. Ineither event, relay R1 closes and feeds power to the vacuum valve V1which opens and admits vacuum to the sealed chamber under the frame. Assoon as vacuum sensing switch S2 detects the preselected vacuum pressurelevel, it closes and passes power to vacuum valve V1, which because ofthe circuitry back-feeding power to the coil of relay R1 locks it andpower to the vacuum valve on. Power is also fed to timer T1, the vacuumcycle length timer, i.e. to both this timers relay's common pole and toits "start timing" (ST) cycle connection.

Timer T1 now begins timing how long the vacuum valve is to be opened. Itshould be understood that timer T1 does not initiate any action untilthe cycle time has elapsed. When the cycle duration has elapsed, thecoil of timer T1's internal relay is energized and three things areeffectuated:

1. Power to relay R1's contacts and coil and thus to the vacuum valve V1is interrupted, thereby terminating the vacuum supply to the frame.

2. Power is now supplied to timer T2, the pressure cycle length timer;this does two things

a. begins supplying power to the pressure valve V2 (thus opening thevalve and flushing the space under the stencil with positive pressure tolift the stencil off the substrate) and

b. begins timing how long the pressure valve will be energized.

3. Power is supplied to the "start timing terminal" of timer T3, theinterrupt duration cycle timer, which times how long it will be beforeanother printing cycle is allowed to be initiated. This interrupt isachieved by running relay R2's coil power lock through the NC contactsof the relay in timer T3. When timer T3's cycle elapses, its relayoperates and those contacts break, de-energizing relay R2, which resetsall circuits. A new cycle may now be initiated.

For manual operation, the interrupt duration timer T3 is set arbitrarilylong enough to allow the printer to make the print and get the screenback up. In automatic printing, the interrupt duration timer is set toallow recycle just before the printing machinery would be setting thestencil down upon a replacement substrate.

Pilot lights L1, L2 and L3 indicate respectively the operation of thevacuum valve timer T1, compressor valve timer T2 and interrupt cycletimer T3. Associated with each timer T1, T2 and T3 are potentiometersRH1, RH2 and RH3 respectively which allow for duration adjustment.Potentiometers RH1 and RH2 might typically have a value of 0-75 K ohmswhile potentiometer RH3 preferably has a range of 0-2.4 M ohms. Relay R2and each of timers T1, T2 and T3 contains a double-pole, double-throwrelay. Relay R1 preferably comprises a single-pole, single-throw relay.Vacuum sensor switch S2 is preferably a single-pole, single-throw,adjustable, pressure differential type switch which closes on sensingthe desired vacuum pressure level. A gauge-cum-switch (such as a DwyerPhotohelic) may be substituted for this switch. The gauge-cum-switchunit has two double-pole, double-throw relays; one could be used as alow-vacuum event switch to feed lock-in power to relay R1 through thecommon pole of timer T1's internal relay, while the other could be ahigh-vacuum event switch to feed timer T1's "start timing terminal" toinitiate the timing cycle. Other modifications and variations in thecontrol unit circuitry and pneumatic subsystem will suggest themselvesto those skilled in this technology.

To assure complete accuracy and controlled layering of the printedimpression, the amount of time the stencil contacts the material to beprinted and the speed of release of the stencil from the printedsubstrate are precisely timed. It is also important to limit thepressure of the vacuum used in printing to the same draw that was usedin photostencil manufacture. These functions are all regulated by thecontrol unit just described.

A detailed description of the total stencil manufacturing and printingprocess, of the present invention, will now be presented.

After the image to be printed has been assessed for definition andlayering requirements, an air distribution frame, as described, isconstructed proportionate to the image area dimensions. Holes aredrilled through the frame for the hose connector(s) and a screen fabricof the specification required is stretched and adhered to the frame. Aspecial finishing process is then, preferably, utilized to coat thephotosensitive emulsion on this screen.

In this process, the bottom of the screen is coated with a thick layerof "direct" emulsion using a soft, rounded squeegee This layer is driedand then an additional layer is applied to the same side using the samesqueegee. Immediately, while this layer is still wet, the screen isplaced, wet face down on a clean smooth sheet of polycarbonate or othernon-hydroscopic material and the squeegee is run over the back of thescreen laminating the sheet of non-hydroscopic material to the screenbottom. When the screen is dry, the sheet of non-hydroscopic material isremoved leaving a screen that is highly flexible and durable with thecontact surface perfectly smooth and polished. This coating procedureassures that the emulsion layer is of a controlled thickness, thecontact surface is of a quality that produces high definition and thestrength of the emulsion hold to the screen is adequate for an extendedproduction life.

This photosensitive screen is then prepared for exposure. Any screenportions between the emulsion and the frame that are still open to airtransmission are securely coated or taped. The outside edge of the frameis banded with the peripheral gasket which can be sealed to the frame'supper side and allowed to lip freely around the underside. A hoseconnector is mounted in one of the holes through the frame and the other(vacuum sensor) hole is sealed. The frame is now fully functional forthe exposure phase of the process.

A clean sheet of glass slightly larger than the frame is chosen. If thematerial to be printed on is to be dimensionally higher than theprinting platen, this thickness in clear material is added to the glassand the film positive of the image is positioned on it. The frame isthen centered over the film positive on the glass and a hose from thevacuum pump, with associated pressure gauge and bleeder valve, isattached to the frame. The vacuum pump is then engaged and the screen isdrawn down to pres against the film positive. The bleeder valve isadjusted so that the draw of the screen is substantially just to theedge of the film positive this pressure is observed on the gauge andnoted for later use in the printing stage. The complete unit: frame withdeflected screen, film positive and glass is then placed facing a lightsource and the image is exposed.

After the image on the screen is developed, dried and any necessarytouch-up done, the frame is connected by a hose connection and sensortube connection to the control unit. The stencil is now ready forprinting.

The developed stencil is no longer sealed to air transmission. Forprinting, the open pores in the image area are sealed by coating themwith ink. The stencil, landened with ink, is placed over a substrate ona printing platen and a vacuum equal to that employed in thephotoexposure phase is created under the stencil. Atmospheric pressuredeflects the stencil to contact and transfer ink to the substrate. Aftera first preselected time period, the vacuum is replaced by compressedair to rapidly release the screen from its printing position. Thecompressed air is activated for a second preselected time period at theclose of which a time duration interrupt occurs to permit substratereplacement.

Referring now to FIGS. 8-11, an alternate embodiment of a printhead orframe 10' is illustrated. In this presently preferred embodiment, frame10' has a two-piece construction and includes an insert 100 and a masterframe 102. As more fully explained hereinafter, insert 100 serves tosupport the tensioned screen/stencil and is readily removable andreplaceable. The master frame receives, secures and supports the insertat a desired off contact distance, and also affords the air distributionfunction.

Insert 100 comprises a substantially rigid continuous frame member tothe underside of which tensioned screen 14' is firmly secured in anyknown fashion. Insert 100 is preferably provided with an outwardlyprotruding ridge 104 extending from its top surface 106. As more fullyexplained hereinafter, ridge 104 helps to retain insert 100 within amaster frame 102 during printhead operation.

Master frame 102 comprises a rigid continuous frame member having aninverted L-shaped cross section. Attached along the underside 108 ofmaster frame 102 is a peripheral gasket 110. Gasket 110 provides acontinuous airtight peripheral seal under master frame 102. This gasketextends measurably below lower surface 108 and serves to define theoff-contact distance at which the periphery of screen 14' is supported.As shown, the gasket can advantageously be provided with an upwardlyextending portion 112 configured to snugly fit within a matching dovetail groove 114 in the base of master frame 102. In this way, gasket 110can be secured to the base of master frame 102 in an airtight relationand lateral movement of gasket 110 inhibited. The outer edge 116 ofgasket 110 is preferably tapered and slightly downwardly inclined, asbest seen in FIG. 11, to ensure a good initial seal. The gasket may bemade of hard rubber or other suitable material such that the portiondirectly below master frame 102 is substantially unyielding and theouter tapered edge 116 is somewhat more flexible. So long as the gasketprovides a continuous airtight peripheral seal and maintains theoff-contact distance, it may be formed of other materials, possess othershapes and be secured by other means to master frame 102.

A series of locking ball plunger mechanisms 118 are located in spacedapart relationship along the perimeter of master frame 102. Each ballplunger mechanism is screw mounted in a respective threaded apertureextending through the upright wall portion 120 of master frame 102. Aball bearing 121 spring loaded at the end of each plunger mechanism fitsbelow ridge 104 and serves to secure the insert within master frame 102during printhead operation. The plunger mechanisms also facilitate readyremoval and replacement of inserts within the master frame. A continuousO-ring type gasket 122 secured within a channel on the underside ofcross leg member 124 of the master frame provides an airtight sealbetween the top surface of insert 100 and the underside of cross legmember 124 of the master frame.

Also attached to cross member 124 of the master frame are hoseconnectors or ports 30' and 32'. Connectors 30' and 32' are threadablysecured in or otherwise suitably mounted to apertures extending throughthe master frame member. The apertures are located in cross member 124between gasket 122 and upright wall 120. Port 30' serves to connect theairtight chamber formed under and along the sides of insert 100 to anexternal source of pressure while port 32 serves to connect the airtightchamber to an external sensor. Depending upon the size of the frame, andthe capacity of the external pressure source, multiple ports 30' may beprovided on master frame 102.

Master frame 102 and insert 104 are preferably made of lightweight,strong material such as aluminum or plastic which retains its structuralintegrity when formed and employed in printhead operation. Insert 100 ismounted within master frame 102 from underneath as shown in FIG. 10, andsecurely held in position therein by the ball plunger mechanisms 118.This mounting arrangement also tends to straighten out any twisting orwarp of insert frame 100 which might be produced by the tensioned screenattached thereto. The spring loaded ball at the end of each plungermechanism has sufficient give to permit entry and removal of the insertframe as desired while also having the capability of securely holdingthe insert frame pressed against gasket 122 when the ball is seatedunder ridge 104 during operation of the printhead. Of course, otherstructures, which perform substantially the same function, may be usedinstead of the ball plunger mechanisms.

Master frame 102 lends rigidity and strength to the overall framestructure. In addition to the locking ball plungers, it incorporates allnecessary gasketing and facilitates the air distribution and pressurelevel sensing functions. The master frame may also provide attachmentpoints for auxiliary clamps or the like which may be used to lift orpivot the frame.

The two-piece construction and particularly the separation of thescreen/stencil supporting function of the insert from the airdistribution function of the master frame provide a number ofsignificant advantages. The principal benefit is that the insert can bereadily removed, replaced and/or disposed of independent of the masterframe. Its simple structure facilitates inexpensive and mass productionof such inserts. At the same time, the master frame can be used for anextended period of time with a host of different inserts. These featuresresult in substantial cost savings and faster set-up. This arrangementalso allows for the ready variation of screen off-contact distance bysimply varying the height of the insert frame.

The two-piece frame can be operated in the same manner, for the samepurposes and achieve the same results, as previously described for theone-piece frame. The master frame and associated insert can thus beadvantageously employed in conjunction with the vacuum source, bleedervalve and pressure gauge, shown in FIG. 3, for stencil manufacturingpurposes and in conjunction with a control unit, such as that shown inFIG. 5, for stencil printing purposes. Alternatively, the frame can beused with the improved control unit 130, illustrated in FIG. 12, forboth the photoexposure and printing phases.

An improved control unit 130 designed to control both screen deflectionduring photoexposure and stencil deflection during printing isillustrated in FIG. 12. The improved control unit, shown with its coverremoved, requires only household current and commonly available shop airto run the printhead thereby eliminating the need for any outside vacuumsource. It employs a manifold absolute pressure sensor capable ofdetecting and indicating both vacuum and pressure levels permittingcompressed air release time to be controlled by a sensed pressurereading rather than a timer. Further the improved control unit employssolid state logic circuitry thereby eliminating the need for relays andelectromechanical timers, enhancing the speed and accuracy of thecontrol function signal processing, and facilitating the packaging ofthe control system as a compact portable unit.

In FIG. 12, improved control unit 130 is depicted as it might be used inconjunction with a two-piece frame 10' for photoexposure purposes. In amanner similar to that earlier described with respect to FIGS. 3 and 4,a stretched screen 14' on an insert frame is coated with a photosensitive emulsion 34 and any peripheral areas of the screen whichremain gas pervious are sealed, for example, with tape 36. The insert isthen mounted within a corresponding master frame, as earlier described,and the two-piece frame placed upon a transparent base 38 of aphotoexposure table so that an emulsion coated image area of the screenoverlies the film positive 40. Control unit 130 is employed tocontrollably deflect the image area of screen 14' into contact with thefilm positive, as illustrated in FIG. 4, and also to activate lightsource 52 to produce photoexposure. The control unit operated in avirtually identical manner and with the same preset vacuum pressurerating can also be used to controllably deflect a stencil into contactwith a substrate, as illustrated in FIG. 6, for printing purposes.

As shown in FIG. 12, frame 10' is connected to control unit 130 viahoses 132 and 134. Hose 132 connects to port(s) 30' and serves as aconduit for selectively evacuating and pressurizing the airtight chamberformed by frame 10'. Hose 134 connects the airtight chamber through port32' to an absolute pressure sensor D1 in control unit 130.

Control unit 130 is connected via port 72 and hose 76 to a remotecompressed air source 80. Knob 86 allows for adjustment of compressedair pressure which is measured by pressure gauge 88. As more fullyexplained hereinafter, no external vacuum source is required sincecontrol unit 130 contains an air driven venturi type vacuum transducer.

Control unit 130 is equipped with a pair of solenoid operated 2-wayvalves V1 and V2. Valve V1 serves to control the vacuum level in theairtight chamber under frame 10' while valve V2 controls the supply ofcompressed air pressure to the airtight chamber. Internal hoses 133connect both valves V1 and V2 to compressed air input port 72.

The pneumatic circuit within improved control unit 130 includes a 3-wayair pilot operated valve 136. The output of valve 136 is connected viahose 132 to frame 10'. The normally open orifice of 3-way valve 136 isconnected via hose 138 to air pressure valve V2. The normally closedorifice of valve 136 is connected via hose 140 to a volume chamber 142.A check valve 144 connects the other end of volume chamber 142 through aventuri 146 and connecting hose 148 to the output of vacuum controlvalve V1. Air pilot input line 150 connects hose 148 to 3-way valve 136and automatically controls the switching of this valve.

The operation of the pneumatic circuit of control unit 130 will now bedescribed. Air received from external compressed air source 80 isregulated via knob 86 and pressure gauge 88 to a constant pressure forall internal operations of the device. This regulated pressure is pipedto the normally closed inlets of solenoid operated valves V1 and V2through internal hoses 133.

A signal transmitted over line 152 from the electronic circuit IC1(described hereinafter) opening valve V1 causes two simultaneous eventsto occur. The regulated air is sent via the air pilot input line 150 toopen the normally closed orifice of the 3-way air piloted valve 136.Simultaneously, the regulated air is sent to the venturi-type vacuumpump 146 to draw a suction against check valve 144 that is normallymaintained closed by atmospheric pressure. The opening of the checkvalve permits the venturi to evacuate the air from volume chamber 142and, through the air piloted opened orifice of the 3-way valve 136, thedown stream sealed chamber of the printhead. When valve V1 is closed,the pressure to the air pilot input line and to the venturi issimultaneously interrupted, thus closing the normally closed orifice ofthe 3-way valve and also allowing atmospheric pressure to close thecheck valve. This causes a vacuum to be trapped between these two valvesin the volume chamber 142. This creates a ballast in chamber 142 whichserves to overcome any lag in the system relative to the creation of thenecessary vacuum at the venturi and the air pilot operating speed. Thesize and/or volume of chamber 142 will vary depending upon thedisplacement of the downstream sealed chamber and connecting hose.

At the end of the photoexposure or print cycle, the regulated pressureis permitted to pass through the normally open orifice of 3-way valve136. This is accomplished by applying a signal from electronic circuitIC1 via line 154 to open valve V2. This action releases the down streamvacuum at the printhead.

The electrical components of control unit 130 will now be described. Asshown in FIG. 12, the control unit includes a single timer T4 connectedto a timer adjustment potentiometer VR1; a first comparator I1 connectedto pressure sensor D1 and to a vacuum pressure adjustment potentiometerVR2; a second comparator I2 connected to pressure sensor D1 and toatmospheric pressure potentiometer VR3; a power supply P1, e.g. avoltage regulating transformer; a solid state logic array IC1; andmaster power switch S3, single-pole, double-throw cycle commence switchS1, and an exposure lamp control switch S4.

The interconnection of the circuit components including a typical solidstate logic array are shown in the schematic of FIG. 13 and will bedescribed in detail hereinafter. But first the general operation of theelectrical system of the control unit will be presented. The circuits inthe control unit are energized by turning master power control switch S3on. A second, cycle control switch S1 is then activated. When switch S1is deflected to its momentary contact position and released, the cyclebegins. Logic circuitry IC1 signals power to the vacuum valve V1 whichopens and admits vacuum to the sealed chamber under the frame. As soonas vacuum sensing device D1 matches the preselected vacuum pressuresetting of potentiometer VR2 at comparator I1, a start signal is fed totimer T4, the vacuum cycle timer and to exposure lamp switch S2. Thetiming period of timer T4 is selected through timer adjustmentpotentiometer VR1. Comparator Il compares the vacuum pressure read bysensor D1 with the desired vacuum pressure set at potentiometer VR2 andcycles valve V1 on and off through the logic gates of circuitry IC1 tomaintain the desired vacuum pressure in the airtight chamber while timerT4 is active. When timer T4 reaches the timer setting established bypotentiometer VR1 it closes valve V1 and deactivates switch S4 whileopening valve V2. This produces a replacement of the vacuum level in theairtight chamber with positive pressure. When sensor D1 determines thatthe positive release pressure from valve V2 has reached the desiredatmospheric pressure reading set through potentiometer VR3, comparatorI2 closes valve V2 and triggers a flip-flop in circuitry IC1 to end thecycle.

Timer adjustment potentiometer VR1 may be of values from 0-100K ohms to0-5M ohms depending on its function as a print duration timer (in themillisecond range) or as an exposure length timer (in the range ofminutes), respectively. The potentiometer for vacuum adjustment VR2 istypically 0-10K ohms and the potentiometer for atmospheric pressureadjustment VR3 is typically 0-25K ohms. Sensor D1 is a solid statemanifold absolute pressure sensor typically found in computer controlledautomobile fuel injection systems. The logic circuit is an array of NANDand NOR gates while the comparators and timer typically comprise linearintegrated circuit packages

The operation of the electrical circuits of control unit 130 will now bedescribed in detail with reference to the schematic of FIG. 13. Theanalog signals of adjustable potentiometers VR1, VR2, and VR3 and sensorD1 are converted to digital high and low signals at the outputs ofcomparators I1 and I2 and timer T4. These digital signals are sentthrough the logic gates of circuitry IC1 to control the process cycle.Circuitry IC1 includes NOR gates N1, N2, N3, N4, N5, N6, N7 and N8,inverter gates I9 and I10, and NAND gate N11 as shown.

When power is switched on, sensor D1 responds to a set variable atpotentiometer VR3 equal to atmospheric pressure causing comparator I2 tooutput a high signal and comparator I1 a low signal. The high outputsignal from comparator I2 responds to a low output signal from timer T4to set the flip-flop N1/N2 and lock valve V2 off through the high outputof inverter gate N10. The low output of NOR gate N3 maintains the outputof timer T4 low and combines with it to drive the output of NOR gate N4high to lock valve V1 off. At this state all interfaces are inactiveexcept for sensor D1.

To start the cycle, switch S1 is switched to its momentary throwposition causing inverter I9 to send a high signal to one input of NORgate N3 and flip-flop N1/N2 to reset and to send a low signal to theother input of NOR gate N3. For the duration of this momentary throwevent, the output of NOR gate N3 remains unchanged from its previouscondition and all interfaces remain inactive. When switch S1 is returnedto its normally closed position, inverter I9 is driven low matching thelow from reset flip-flop N1/N2 and NOR gate N3 outputs a high. This highsignal is sent to NAND gate N11 to arm the trigger of timer T4 and toNOR gate N4 to change its output to NOR gate N5 to a low signal. SensorD1, responding to a set variable at adjustable potentiometer VR2 equalto a percentage of vacuum between the atmospheric pressure setting ofpotentiometer VR3 and absolute vacuum, will output a low at the outputof comparator I1 until the sensed value of D1 equals the set reading ofpotentiometer VR2. This low is sent to NOR gate N5 and NAND gate N11. AtNOR gate N5 it causes valve V1 to be activated; at NAND gate N11 itholds timer T4 in an armed state. Sensor D1, responding to a setvariable at potentiometer VR3, reacts to the pressure drop from anactivated valve V1 and changes the output of comparator I2 from high tolow while the sensed value at sensor D1 is not equal to the set readingat potentiometer VR3. This sends a low to one input of NOR gate N7 whichremains low at its output until timer T4 has been triggered.

At the moment that the sensed value of D1 equals the vacuum pressuresetting at potentiometer VR2, comparator I1 outputs a high whichsimultaneously deactivates valve V1 and triggers timer T4. Thetriggering of timer T4 sends a high from T4 to NOR gates N4, N6 and N8and exposure lamp switch S4. At NOR gate N8 the signal causes a lowoutput that simultaneously causes inverter I10 to output a low outputsignal to NOR gate N6 and also causes flip-flop N1/N2 to be set. Thisset state in turn causes the output of NOR gate N3 to return to lowwhich serves to prevent the re-triggering of timer T4 and to put a lowinput at NOR gate N4 to drive the output of N4 low. This low output ofNOR gate N4 while timer T4 remains high allows a slight rise in thepressure at sensor D1 to continually reactivate valve V1 throughcomparator I1 when the value at sensor D1 is not equal to the reading atpotentiometer VR2, and de-activate valve V1 as the sensor value equalsthe preset reading at potentiometer VR2 to maintain a precise pressuresetting throughout the timer cycle. (The cycle time or period of timerT4 is set at potentiometer VR1.)

At the moment that the timer cycle is completed, timer T4 outputs a lowthat simultaneously: de-activates valve V1 for the remainder of thecycle by causing NOR gate N4 to output a high; de-activates lamp switchS4 for the remainder of the cycle; and activates valve V2 by causing NORgate N6 to output a high. The positive pressure of valve V2 returns thesensed value at sensor D1 to the set variable of potentiometer VR3 (i.e.atmospheric pressure) causing comparator I2 to output a high. This highsignal drives NOR gate N7 low and, in turn, drives inverter I10 outputhigh and NOR gate N6 output low to de-activate valve V2 and return theentire circuit to the same state as at the beginning.

In summary, control unit 130 provides automatic adjustable time andpressure control over compressed air and vacuum which can be used forboth the stencil manufacturing and the stencil printing phases of thepresent invention. The photoexposure or printing cycle begins withswitch S1 opening valve V1 which in turn supplies vacuum to draw downthe screen/stencil. When the predetermined vacuum pressure level set atpotentiometer VR2 is detected by sensor D1, vacuum pressure valve V1 isclosed and timer T4 commences timing the photoexposure or vacuum printinterval during which time the logic circuitry IC1 cycles valve V1 openand closed to maintain the precise vacuum setting of potentiometer VR2for as long as the timer T4 is activated. At the conclusion of theselected time interval, the vacuum valve V1 closes and the logiccircuitry IC1 begins the opening of pressure valve V2 to rapidly releasethe screen/stencil from the substrate until sensor D1 senses a positivepressure equal to the setting at potentiometer VR3, i.e. equal toatmospheric pressure. At atmospheric pressure, valve V2 is automaticallyclosed and the cycle is interrupted through the logic gates of IC1 untilanother commence signal is received through switch S1. In a manualprinting mode, the printing cycle is initiated by the operator pushingthe spring-loaded momentary switch S1; in automatic operation, switch S1can be replaced with a common proximity sensing switch on a typicalconveyorized delivery device.

Accordingly, improved control unit 130 is usable in both the stencilpreparation and stencil printing phases, optimally implements thematched deflection process of the present invention, affords precisecontrol over screen/stencil deflection and release for any selectivedeposition need, is packagable as a compact, lightweight, desktop unitand makes possible automatic photoexposure. This latter feature may beutilized to perform other exposure functions. For example, the vacuumcontrol potentiometer could incrementally adjust the deflective membraneof a typical lithographic plate exposure vacuum frame until a presetmaximum level of vacuum was obtained for the purpose of eliminatinghalation-causing pockets of trapped air.

For maximum benefit, control unit 130 can be used with a 2-componentprinthead such as that shown in FIG. 8. Alternatively, the control unitcan be employed with a unitary frame or one of other construction. Whenutilized together, the printhead and control unit of the presentinvention allow for extremely rapid, high quality printing. Theequipment can be beneficially applied to the fields of marking,decorative art, electronic component manufacturing, material dispensingand wherever else a controlled deposition of material is desired.

It will thus be apparent that the present invention provides a solutionto the distortion problems associated up until now with "silk screen"printing and facilitates such printing with a speed and accuracyheretofore unobtainable. With solid state sensors and circuits, timesettings are practical in the millisecond range depending upon deliverydevice (e.g. conveyor) cycle rates and the internal limits of the valvemechanical components. With the present invention it is estimated thatprinting may be possible at speeds in the 10 impressions per secondrange or 36,000 impressions per hour. At least as important as theimproved cycle rate is the added accuracy and controllability of thequality of the individual impression produced. With the presentinvention, images having lines as fine as 25-30 microns wide and spacesbetween the lines on the order of 45 microns can be accuratelyreproduced. The present invention is thus particularly beneficial to theelectronics industry and other applications requiring fine printing,close tolerances or minimal distortion.

Although the invention has been described in terms of its presentlypreferred embodiment, modifications, substitutions and variations may bemade within the scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. Apparatus for use in stencil manufacturing andprinting comprising, in combination:A. A frame comprising:frame meansfor peripherally supporting a stretched and sealed screen member in afirst plane; a gasket means attached to said frame means and extendingbeyond said first plane for forming a peripheral air tight seal when theframe rests on a supporting surface and for continuously maintaining theperiphery of said screen member at a fixed off-contact distance fromsaid supporting surface; first port means secured to said frame meansfor facilitating connection of a chamber bonded by said screen member,said supporting surface and said gasket means to an external source ofvacuum pressure; and vacuum sensing port means secured to said framemeans for selectively connecting said chamber to vacuum pressure sensormeans; B. a compressed air source; and C a control unit comprising:meansfor controlling the vacuum pressure in said chamber; vacuum pressuresensor means for automatically determining when a desired partial vacuumpressure level exists in said chamber; means for automaticallymaintaining said desired partial vacuum pressure level in said chamberfor a first preselected period of time, whereby a central portion ofselected variable area of a gas impervious screen member supported bysaid frame means can be controllably deflected from said first plane tocontact the substrate located on said supporting surface by selectivelyreducing the pressure in said chamber to a desired partial vacuumpressure level detected by said pressure sensor means; means forconnecting said compressed air source to said chamber, upon expirationof said first preselected period of time, for a second period of time,whereby a quick release of the deflected portion of the screen memberfrom the substrate can be effected; common hose means connecting saidcontrol unit to said first port means for alternately conveying vacuumpressure and compressed air from the control unit to said chamber; meansfor establishing a desired atmospheric pressure level and fordisconnecting said compressed air source from said chamber when thepressure in said chamber detected by said sensor means reaches thedesired atmospheric pressure level, the vacuum pressure sensor meansalso detecting positive pressure levels; a venturi type vacuum pressuretransducer for converting compressed air into vacuum pressure; a 3-wayair pilot operated valve for alternately supplying vacuum pressure andcompressed air to said common hose means; and means for independentlyvarying the desired vacuum pressure level, first preselected period oftime, and desired atmospheric pressure level.